1. Field of the Invention
The present invention relates to a relatively large rectangular sealed alkaline storage battery and a module battery comprising the collected storage batteries.
2. Prior Art
At present, as for large storage batteries, lead acid storage batteries are used as batteries for SLI of automobiles and stationary batteries. As electric sources for small home equipments, nickel-cadmium storage batteries are used and recently, alkaline storage batteries such as nickel-metal hydride storage batteries superior in energy density have spread.
In recent years, environmental disruption has rapidly advanced on a global scale and as one cause thereof, waste gases from automobiles have become a problem. In order to cope with the problem, development of electric cars has been hastened and development of electric sources holds the key to the attainment of the development of the electric cars.
As these driving electric sources, lead acid storage batteries are mainly used at present. However, since the lead acid storage batteries are low in energy density per unit weight, one charge distance range is very short. Furthermore, since these batteries are of an open type, troublesome maintenance such as electrolyte addition is needed. Furthermore, nickel-cadmium storage batteries or nickel-iron storage batteries high in energy density and long in one charge distance range are also partially used as electric sources for electric cars, but these storage batteries are also not of a sealed type and need troublesome maintenance such as electrolyte addition.
From the viewpoint of safety, too, sealed batteries which require a minimum amount of an electrolyte are desired.
On the other hand, since the internal pressure of sealed alkaline storage batteries increases owing to the gas generated at the time of charging and discharging, a safety vent is provided at the sealed container and when this safety vent operates and mist of electrolyte is released outside together with the gas, the battery capacity decreases due to decrease of the amount of an electrolyte.
To solve these problems and realize increase of one charge distance range and maintenance-free batteries, development of new storage batteries of a high energy and completely sealed type is demanded.
The above storage batteries when subjected to charging and discharging cause expansion and shrinkage of electrodes. When the structure is of rectangular sealed type, the group of expanded electrodes is pressed by the container wall to cause distortion of plates and the distance between the facing plates becomes non-uniform and the active material partially falls off in case there is no sufficient space between the container inner wall and the shorter side face or the bottom of the electrode group.
For this reason, the electrode reaction becomes non-uniform and the active material cannot be effectively utilized, causing reduction of discharge capacity, rise of the internal pressure of the batteries during charging and internal short-circuit. Furthermore, if there is no space in the container containing the electrode group, it is difficult to carry out rapid filling of a given amount of an electrolyte. Moreover, if the amount of the electrolyte to be filled is insufficient for the reaction capacity of a nickel cathode, utilization and cycle life of the batteries decrease. If the amount of the electrolyte is too much, the reaction at the anode for absorbing oxygen gas generated from the cathode in the case of overcharging is hindered and the mist of electrolyte leaks through the safety vent together with the gas to result in reduction of cycle life.
Normally, the electrode group is composed of piled electrode plates. Therefore, unless the electrode group is constrained with a band or the like, operability for insertion of the electrode group into the container is low. The expansion of the electrode group is conspicuous in the non-constrained direction or in the downward direction of the electrode plates where the constraint is weak. Therefore, in case the length of the lower part of a bag-like separator which covers the electrode plate has no margin and further, the lower part of the bag-like separator is closed, for example, by fusion bonding, the lower part of the separator is broken by the expanded plate and there are also the possibility of falling off of the active material to the bottom of the container and the possibility of internal short-circuit.
Furthermore, if the electrode plate is completely covered up to the top part thereof by the separator, the gas generated at charging and discharging stays in the separator and the battery may rupture due to rising of gas pressure. Moreover, in case the container is made of a synthetic resin, when the electrode plate group expands in the direction of the longer side face of the container, the container itself is distorted and consequently, there is no sufficient space between the shorter side face of the electrode group and the inner wall of the container.